Minimally-invasive surgery has evolved to a point where procedures that were unimaginable a few years ago are now routinely performed on a daily basis. Indwelling ureteral stents have been widely used for years. Such stents are placed in the ureter, which is the duct between the kidney and the bladder, for the purpose of establishing and/or maintaining an open, patent flow of urine from the kidney to the bladder.
Nephrolithiasis is a condition in which one or more calculi or stones are present in the kidneys. Kidney stones affect 3-5% of the world population and 80% of the stones are calcium-based; calcium oxalate being predominant. Other stones are made up of calcium phosphate, uric acid, cystine or struvite (magnesium ammonium phosphate). Small crystal aggregates are typically excreted from the body via passage in urine. Once the aggregrates grow to a certain size (3 mm) however, they can cause obstruction of the ureters due to urolithiasis.
Ureteral stents may be used to retain patency of the ureteral lumen and to continue normal urinary drainage following the treatment and removal of stones and calculi from kidneys and ureters. To treat this condition, several individual steps are involved. In one procedure, these steps include placing a relatively narrow wire guide through a urethra and bladder, and then through the ureter and into the kidney. After the wire guide is placed, a catheter is run along the wire guide, dilating the body passage (the urethra and the ureter) as it moves down the wire guide. The access sheath also dilates the body passages as it moves from outside the body, through the urethra, and into the ureter, down the desired location, and into or very near the kidney.
The physician may then remove calculi and stones through the access sheath, using a grasper, a retrieval basket or other device. The access sheath protects the ureter from repeated passage of the retrieval device while the stones or calculi are removed. After the stones are removed, the ureteral stent may be placed into the ureter through the access sheath, using the catheter or a pushing tube to position the stent.
Lifetime risk of nephrolithiasis in the developed world is 10-15%, and is higher elsewhere (20-25% in the Middle East). In the US, nephrolithiasis is estimated to occur in up to 10% and 5% of the male and female population respectively, result in medical costs of $2.1 billion annually. 70% of those who experience nephrolithiasis will have recurrence of stones even after surgical removal. Once recurrent, the subsequent risk of relapse is raised and the interval between recurrences is shortened. Crystallization of calcium and magnesium salts is the chief culprit in the formation of kidney stones.
The typical ureteral stent can be composed of various radiopaque polymers, including polyethylene, silicone, polyurethane, and thermoplastic elastomer. These stents are retained in the ureter by a retentive anchoring portion, such as a curved shape, pigtail, coil, J-shape, or hook configuration, at either end of the stent that engages the walls of the bladder and the kidney, respectively. The stent is resilient to allow it to be straightened for insertion into a body passageway and returned to its predetermined retentive anchoring shape when in situ. There can be problems, however, with ureteral stents, as urine may fail to drain through the stent. This may be due to a number of reasons, such as extrinsic compression of the stent or blockage of the drainage mechanism of the stent by encrustation. Furthermore, there can be problems associated with migration of the ureteral stent from the original implantation site either upward into the kidney of the patient or downward into the bladder of the patient.